Various state differences notwithstanding, to establish a prima facie case of defamation, one must show:
1) a false statement purporting to be fact; 2) publication or communication of that statement to a third person; 3) fault amounting to at least negligence; and 4) damages, or some harm caused to the person or entity who is the subject of the statement.
Additionally, since The Donald is a public figure and public figures often get publicly talked about, one must, per the famous NY Times v. Sullivan SCOTUS case, show
that the false, defaming statements was said with "actual malice." The Sullivan court stated that"actual malice" means that the defendant said the defamatory statement "with knowledge that it was false or with reckless disregard of whether it was false or not." The Sullivan court also held that when the standard is actual malice, the plaintiff must prove actual malice by "clear and convincing" evidence, rather than the usual burden of proof in a civil case, which is the preponderance of the evidence standard.
Sorry, forgot to actually tie this back to your question.
To prevail, Trump would have to prove - by clear and convincing evidence, which is harder to achieve than preponderance - that, essentially, CNN conducted this poll, received results X, but instead purposely reported results Y.
Per your question, assuming the above, then theoretically, maybe he could prevail. No, it does not have to be about something he did or didn’t do. Another example could be publishing a news report that someone is a pedophile. That is not necessarily about something someone did or didn’t do.
In the poll example, especially considering how often polls are wrong and this far away from the election, the tougher hurdle to clear for Trump would be to prove there were damages.